Real-Time Motion Capture Toolbox (RTMocap): an open-source code for recording 3-D motion kinematics to study action–effect anticipations during motor and social interactions

Abstract

We present here a toolbox for the real-time motion capture of biological movements that runs in the crossplatform MATLAB environment (The MathWorks, Inc., Natick, MA). It provides instantaneous processing of the 3-D movement coordinates of up to 20 markers at a single instant. Available functions include (1)the setting of reference positions, areas, and trajectories of interest; (2)recording of the 3-D coordinates for each marker over the trial duration; and (3)the detection of events to use as triggers for external reinforcers (e.g., lights, sounds, or odors). Through fast online communication between the hardware controller and RTMocap, automatic trial selection is possible by means of either a preset or an adaptive criterion. Rapid preprocessing of signals is also provided, which includes artifact rejection, filtering, spline interpolation, and averaging. A key example is detailed, and three typical variations are developed (1)to provide a clear understanding of the importance of real-time control for 3-D motion in cognitive sciences and (2) The Real-Time Motion Capture (RTMocap) Toolbox is a MATLAB toolbox dedicated to the instantaneous control and processing of 3-D motion capture data. It was developed to automatically trigger reinforcement sounds during reachand-grasp object-related movements, but it is potentially useful in a wide range of other interactive situations-for instance, when directing voluntary movements to places in space (e.g., triggering a light on when the hand reaches a predefined 3-D position in a room), when performing actions toward stationary or moving objects (e.g., triggering a sound when an object is grasped with the correct body posture), or simply as a way to reinforce social interactions (e.g., turning music on when a child looks at a person by moving the head in the proper direction). The RTMocap Toolbox is created from open-source code, distributed under the GPL license, and freely available for download at http://sites. google.com/site/RTMocap/.The RTMocap Toolbox is mainly intended to work with recordings made with an infrared marker-based optical motion capture system. Such motion capture systems are based on an actively emitting source that pulses infrared light at a very high frequency, which is then reflected by small, usually circular markers attached to the tracked body parts and objects. With each camera capturing the position of the reflective markers in two dimensions (Fig. 1), a network of several cameras can be used to obtain position data in 3-D. The RTMocap Toolbox was developed with the Qualysis motion capture system, but it can be adapted, by means of slight changes in code lines, to other 3-D motion capture systems that provide real-time information in the MATLAB environment-for instance,

Individual differences in reading social intentions from motor deviants

“…A second point to note is the importance in future studies to assess whether the results presented here can be generalized to more ecological tasks. Indeed, the method presented here using video clips could be further applied to create experimental situations at second-person perspective including, for instance, two participants performing a reach to grasp task in a real interactive situation (see illustrated examples on line through reference keys given in Lewkowicz and Delevoye-Turrell, 2015 ). Furthermore, demonstrating that similar patterns of results are obtained when not only two but multiple intentional possibilities are presented would provide more ecological validity for the social abilities reported in the present study (see Obhi, 2012 ).…”

“…Movies were compressed with FFdshow codec (MJPEG) at 30 frames per second with a screen resolution of 640 × 480 pixels. 3D kinematics were analyzed with RTMocap toolbox ( Lewkowicz and Delevoye-Turrell, 2015 ). Positional data points were filtered using a dual fourth-order Butterworth low-pass filter (fc = 15 Hz; forward and backward) and tangential 3D instantaneous velocities were calculated.…”

“…Hence, to characterize the kinematic deviance due to social intention one needs to disentangle the multiple kinematic features to determine the potential candidates. By controlling precisely the external constraints of executed movements in real-time ( Lewkowicz and Delevoye-Turrell, 2015 ), it is possible to verify that the significant deviances of kinematic features are not due to specific motor constraints but rather to internal determinants (see also Ansuini et al, 2015 ), which would give scientific basis for a better understanding of the Dr. Jekyll and Mr. Hyde paradox ( Jacob and Jeannerod, 2005 ). Whereas it has already been shown that the early deviants of kinematic features could be directly exploited to help detect the underlying intention of an observed action ( Sartori et al, 2011 ; Lewkowicz et al, 2013 ), it is still unclear whether the sensitivity to kinematic deviances is in relationship with the motor expertise or the social skills of the external observer.…”

The combined effects of motor and social goals on the kinematics of object-directed motor action

“…Motor performances recorded by the Qualisys system were processed by an in-house script adapted from the RTMocap toolbox for Matlab 58 . In line with the existing literature 34,35,38 , we analyzed only the trajectory of the wrist marker placed at the level of the scaphoid, which expresses arm movements without including wrist rotation.…”

Predictable real-time constraints reveal anticipatory strategies of coupled planning in a sequential pick and place task

“…If any marker was occluded on more than 20 consecutive recorded frames (100 ms), the trial was excluded from the analysis and placed systematically at the end of the block for participants to repeat. The time delays of the real-time control have been estimated previously to be under 10 ms (Lewkowicz & Delevoye-Turrell, 2016). This is important as it insures a reliable recording frequency of 200 Hz.…”